1. The molecular organization of simian sarcoma virus (SSV) cloned DNA by R-loop analysis revealed that this virus was generated by genetic recombination of helper simian sarcoma associated virus and about 1 kilobase pairs of cellular sequences. Construction of deletion mutants of SSV showed excellent correlation between physical localization of acquired sequences and its functional role in cellular transformation. 2. Rauscher murine leukemia virus and Moloney murine leukemia virus were shown to possess similar genomic complexity and thus their specificity in inducing B or T cell tumorigenesis may be a result of yet unknown mechanism. 3. Molecularly cloned BALB-murine sarcoma virus and Harvey-sarcoma virus DNA were shown to possess a homologous region of 0.75 kilobase pairs in their acquired transformation-specific sequences which play an important role in tumorigenesis. These results show that these two independent sarcoma virus isolates have been generated by acquisition of similar subsets of cellular genes as a result of genetic recombination and their transforming function may involve a similar molecular mechanism.